1. Field of the Invention
The present invention relates to an optical switching method and an optical switch, and in particular to an optical switching method and an optical switch using a nonlinear optical effect which occurs in a nonlinear medium.
2. Description of the Related Art
With an optical fiber communication having recently increased in capacity, a bit rate of a transmission system has reached 40 Gb/s, and researches and developments for transmitting an optical signal of equal to or more than 160 Gb/s per wavelength have been carried out in research of a next-generation system. For researches and developments of such a transmission system, an optical waveform measuring device (sampling oscilloscope) which measures the waveform of a signal light becomes essential for monitoring/evaluating the quality of the signal light.
In order to measure the waveform of the signal light by the optical waveform measuring device, an optical switch for appropriately sampling the signal light is used. In the prior art optical waveform measuring device has measured the optical waveform by firstly performing an opto/electro conversion to the optical signal with an opto/electro converter and then electrically sampling the converted electric signal with an electronic circuit.
In such a prior art optical waveform measuring device, signal processing is performed all by an electric circuit after the stage the opto/electro conversion. Therefore, the performance thereof is restricted by an electric signal processing speed. For example, if the electric circuit has an electric signal processing speed (bandwidth) of 40 GHz to switch the signal light of equal to or more than 40 Gb/s, the signal light whose bandwidth exceeds 40 GHz can not be accurately switched.
Contrarily, there has been an optical signal processing technology for performing signal processing by controlling an optical signal by another optical signal with the nonlinear optical effect (parametric amplification) which occurs in the nonlinear medium (see e.g. the following patent document 1).
A response speed of the nonlinear optical effect is said to be on the order of femtosecond, and the signal processing speed of the optical switch applying the effect greatly exceeds the processing speed of the above-mentioned electric signal. If this optical switch is applied to the optical waveform measuring device, the optical waveform measuring device which can observe even a signal light of Tb/s can be realized.
FIG. 10 is a block diagram showing an arrangement of such a prior art optical switch. As shown in FIG. 10, a prior art optical switch 800 has a polarization adjuster 801a, a polarization adjuster 801b, an optical coupler 802, a highly-nonlinear fiber (nonlinear optical medium) 803, a polarizer 804, and an optical bandpass filter 805.
The polarization adjuster 801a adjusts a polarization direction of a signal light Os inputted to a polarization direction (polarization state) inclined by 90 degrees with respect to a passing axis of the polarizer 804. The polarization adjuster 801b adjusts a polarization direction of a control light OP (sampling pulse) inputted to a polarization direction inclined by approximately 45 degrees with respect to the passing axis of the polarizer 804. The optical coupler 802 couples or multiplexes the signal light OS with the control light OP whose polarization directions are respectively adjusted by the polarization adjusters 801a and 801b. The highly-nonlinear fiber 803 passes the signal light OS and the control light OP coupled by the optical coupler 802, thereby generating an intensity correlation signal (cross phase modulation signal) of the signal light OS and the control light OP.
The polarization direction of the signal light OS is changed by the nonlinear optical effect in the highly-nonlinear fiber 803, and the signal light OS is outputted in the polarization direction approaching to the polarization direction of the control light OP. The polarizer 804 has the passing axis of the signal light OS and the control light OP in a predetermined direction, and passes therethrough only polarization components in the polarization direction in parallel with the passing axis. The optical bandpass filter 805 passing therethrough only a wavelength λs of the signal light OS passes and outputs only the polarization component of the signal light OS among the polarization components of the signal light OS and the control light OP having passed through the polarizer 804.
The above-mentioned operation will now be more specifically described referring to FIGS. 11A-11C. It is to be noted that the direction and the size of the arrow corresponding to the signal light OS shown in FIGS. 11A and 11B indicate the polarization direction and the amplitude of the signal light OS. Also, the polarization direction of the signal light OS by the polarization adjuster 801a is, as shown in FIG. 11A, orthogonal to a polarization main axis direction “y” of the polarizer 804. Furthermore, FIG. 11C schematically shows a switching operation by the optical switch.
Firstly, during the period without the control light OP, neither an optical parametric amplification nor a cross phase modulation occurs in the nonlinear optical fiber 803. Therefore, a polarization direction of a signal light OS0 (see FIG. 11B) outputted from the nonlinear optical fiber 803 is the same as that of the input end thereof. Namely, the present polarization direction of the output signal light OS0 is orthogonal to the polarization main axis direction “y” of the polarizer 804, so that the signal light OS0 is completely interrupted by the polarizer 804.
On the other hand, with the control light OP being provided, the signal light OS is amplified with optical parametric amplification by the nonlinear optical fiber 803, and the polarization direction is changed by the cross phase modulation. However, the power of the control light OP is extremely large, so that the signal light OS becomes a signal light OS1 (see FIG. 11B) amplified with the optical parametric amplification due to optical four-wave mixing. The efficiency of the optical parametric amplification becomes maximum when the polarization direction of the control light OP coincides with that of the signal light OS as shown in FIG. 11B.
Also, a signal light component newly generated by the optical four-wave mixing for the signal light OS1 in the polarization state which coincides with that of the control light OP is not affected by the cross phase modulation, so that the polarization direction is not changed by the cross phase modulation. Accordingly, the polarization direction of the signal light amplified with the optical parametric amplification in the nonlinear optical fiber 803 is fixed in approximately the same direction as that of the control light OP as shown in FIG. 11B, and the polarization direction never rotates in excess of 90 degrees.
The angle between the polarization direction of the signal light OS at the input terminal of the nonlinear optical fiber 803 and the polarization direction of the control light OP is set at approximately 45 degrees. Also, the angle between the polarization direction of an output signal light OS2 and the polarization main axis direction of the polarizer 804 is approximately 45 degrees. Accordingly, approximately 50 percent (=(1/√2)2) of the power of the signal light outputted from the nonlinear optical fiber 803 passes through the polarizer 804.
In such an arrangement, a timing of inputting the control light OP into the highly-nonlinear fiber 803 is controlled, so that the switching of the signal light is performed. Also, when this optical switch 800 is applied to the light waveform measuring device, the opto/electro conversion is performed to the signal light outputted from the optical switch 800 by a photo detector, and the waveform of the electric signal converted is displayed, thereby measuring the waveform of the signal light.    [Patent document 1] Japanese patent application laid-open No. 2006-184851
However, in the above-mentioned prior art, it is premised for stably and accurately switching the signal lights that a signal light and a control light whose polarization states are respectively stable are inputted. Accordingly, there has been a problem that a signal light in a state where the polarization state fluctuates in propagation through an optical fiber or the like can not be accurately switched. Also, there has been another problem that the waveform of the signal light can not be accurately measured since the signal light can not be accurately switched.
On the other hand, it is conceived that the polarization state of the signal light whose polarization direction is adjusted by the polarization adjuster 801a is monitored, and the adjustment of the polarization direction of the signal light based on the monitored result is controlled in a feedback mode.
However, there is still a case where fluctuations of the polarization state of the signal light occurs within the optical switch 800 occur due to the fluctuation of the polarization state of the signal light inputted to the optical switch 800 and external environment changes such as changes of temperature and vibration, so that the polarization state of the signal light upon passing through the polarizer 804 can not be accurately monitored. In this case, there has been a further problem that the polarization direction of the signal light can not be appropriately adjusted, so that the signal light can not be accurately switched.